How do I de-register my ereader from an adobe account I no longer have access to?

I have an adobe account set up with my old email address I can no longer access. My ereader is registered to my old account (on my dead computer) and now i can't put the new books I've bought onto my reader until I have 'registered' the reader with my new account - but it won't let e because the serial number isn't valid. I assume this is because it's still registered to my old account.